INCONEL.RTM. alloy 617 (trademark of assignee) is a solid solution nickel-chromium- cobalt-molybdenum alloy exhibiting excellent high temperature strength and resistance to oxidizing and reducing environments. See U.S. Pat. No. 3,859,060. The alloy displays excellent resistance to a wide range of corrosive environments and is readily formed by and welded by conventional techniques. It is used, amongst other places, in demanding petrochemical applications where it generally provides excellent service.
The nominal chemical composition of INCONEL alloy 617 is shown below (in weight per cent).
______________________________________ BROAD PUBLISHED ______________________________________ Nickel Bal 52 Chromium 20-24 22 Cobalt 9.5-20 12.5 Molybdenum 7-12 9.0 Iron 1.5 Alumium 0.8-1.5 1.2 Carbon .ltoreq..15 0.1 Manganese 0.5 Silicon 0.5 Titanium 0.3 Copper 0.2 ______________________________________
The alloy is, however, susceptible to intergranular polythionic acid stress corrosion cracking. Cracking caused by polythionic acid can occur in the annealed condition or after long term exposure at simulated operating temperatures up to about 649.degree. C. (1200.degree. F.). Polythionic acid may be present in petrochemical environments. Failure of components made from INCONEL alloy 617 due to this or any other condition clearly cannot be tolerated.